1. Field of the Invention
The present invention relates to an improvement in a planetary gear apparatus having an internal gear provided with inner teeth which are constituted by pins or a combination of pins and rollers, and an external gear provided with outer teeth having, for example, a trochoidal tooth form. More particularly, the present invention pertains to an improvement in a planetary gear apparatus having a joint which is constituted by a universal joint.
2. Description of the Related Art
A typical conventional planetary gear reducer will first be described hereinunder, with reference to FIGS. 5 and 6.
The illustrated known planetary gear reducer is arranged so that a rotational force imparted from an input shaft 1 is transmitted to an output shaft 2 after its rotational speed has been reduced.
The mechanism for reducing the speed is as described below.
Eccentric members 3.sub.1 and 3.sub.2 are provided on the input shaft 1 at a predetermined phase difference (180.degree. in this example). The eccentric members 3.sub.1 and 3.sub.2 are in contact with external gears 5.sub.1 and 5.sub.2, respectively, through rollers 4. Each of the external gears 5.sub.1 and 5.sub.2 is provided with a plurality of inner roller bores 6. An inner pin 7 and an inner roller 8 are fitted into each of the bores 6. The outer periphery of each of the external gears 5.sub.1 and 5.sub.2 is provided with outer teeth 9 having, for example, a trochoidal or circular form. The outer teeth 9 engage with outer pins 11 provided on an internal gear 10.
The inner pins 7 are shrink fitted into a flange 12 of the output shaft 2.
In this known art, when the input shaft 1 rotates one revolution, the eccentric members 3.sub.1 and 3.sub.2 also rotate one revolution, but the external gears 5.sub.1 and 5.sub.2 are prevented from self rotating because of the engagement between the inner roller bores 6 and the inner pins 7. Therefore, if the difference between the number of teeth on the external gears 5.sub.1 and 5.sub.2 and the number of outer pins 11 (acting as inner teeth) is one, when the input shaft 1 rotates one revolution, the external gears 5.sub.1 and 5.sub.2 move relative to the internal gear 10 only by an amount corresponding to one outer tooth 9. In consequence, one revolution of the input shaft 1 is reduced to an amount through which the external gears 5.sub.1 and 5.sub.2 turn, which corresponds to the reciprocal of the number of teeth on the gears 5.sub.1 and 5.sub.2, and the rotational force whose speed is thus reduced is then transmitted to the output shaft 2 through the inner pins 7.
The planetary gear mechanism of the type described above, however, retains the following disadvantages.
The power transmission mechanism which is constituted by the inner roller bores 6, the inner pins 7 and the inner rollers 8 applies a radial force to the output shaft 2. It is therefore necessary to provide a flange 12 which is rigid enough to prevent the output shaft 2 from being deformed by the application of that force. For this reason, the size of members or portions near the output shaft 2 cannot be reduced in the diametrical direction of the shaft 2. In addition, the power transmission mechanism requires highly-accurate machining and must also have a suitably high rigidity, since any slight deformation of a member or portion thereof has an adverse effect on the operation.
In addition, the inner roller bores 6 occupy a relatively large space in the external gears 5.sub.1 and 5.sub.2, and therefore hinder any reduction in size of the external gears 5.sub.1 and 5.sub.2, which impedes any reduction in the size of the apparatus as a whole.
Another type of known mechanism (see the specification of Japanese Patent Application Laid-Open No. 146,939/85) is arranged so that a carrier is mounted on an external gear and is connected to an output shaft by an intermediate drive gear, whereby radial load is prevented from acting on the output shaft. In this apparatus, however, the input shaft is supported in a cantilever fashion, and this means that the bearing of the shaft must withstand a relatively large load, so that it is not possible to reduce the size of the casing housing the elements on the input-shaft side.